There were no specs for the MySQL adapter in dm-migrations so I
created the spec file. I also mimicked those from sqlite3 for the
core functionality.

I then added the class TableModifier with the method
rename_column. The method extracts the existing column from the
table columns, updates it's name, and then generates the correct
ALTER TABLE statement.

I hope it is ok that both of these are in one patch, but the
later relies on the former!

This is my first patch, so sorry if it's all wrong, but let me
know what any problems are and I'll try to fix them.

When I attempted to fix this bug I found numerous problems with
dm-migrations. I think dm-migrations should be queued up for a
rewrite before DM 1.0 is released, since it's one of the weakest
official DM libs.